Polymerized solutions for depositing optical oxide coatings

ABSTRACT

A clear solution is prepared by reacting metal alkoxide with a mixture of critical amounts of water and/or acid in an alcohol diluted medium. Alkoxides may be Ti(OR) 4  or Ta(OR) 5 , or another metal alkoxide such as Si(OR) 4  in admixture with these alkoxides. Acids may be HCl or HNO 3 . Quarter wave inorganic optical coatings are deposited by applying the alkoxide solution to a substrate then heating the coating at over 350° C. The coatings reduce reflectivity on silicon solar cells. The index of refraction of the coating can be varied by several techniques, including altering the proportion of titanium and silicon in the coating firing temperature, firing atmosphere. Thicknesses of the coating can be controlled by varying the rpm in spin application, withdrawal rate in dipping application, by concentration of the solution, by the type of solvent or the degree of polymerization of the titanium complexes.

This is a division of application Ser. No. 065,706, filed Aug. 10, 1979,which was a continuation of Ser. No. 931,346, filed Aug. 8, 1978, bothnow abandoned.

BACKGROUND OF THE INVENTION

Due to the shortage of hydrocarbon fuels the importance of silicon solarcells as an energy source has increased greatly in the last few years.The efficiency of silicon in absorbing solar radiation, however, is onlyabout 60%, the other 40% being reflected back into the atmosphere. Theamount of light reflected can be reduced by coating the silicon with amaterial having an index of refraction between that of silicon and airaccording to Fresnel's equation: ##EQU1## where n₀ is the index of air,n₁ is the index of the coating, and n₂ is the index of the silicon.

Thus, for silicon, which has an index of about 4, the coating shouldhave an index of about 2.0. (If the coating is applied on a slightlyoxidized surface, the index may have to be adjusted.) This maximumreduction in the amount of light reflected is achieved at a coatingthickness of one quarter of a wavelength.

Presently, it is known how to deposit coatings having specific indicesof refraction. For example TiO₂ coatings have an index of 2.5 to 2.7,Al₂ O₃ coatings have an index of 1.76, SiO₂ coatings of 1.55, etc. Inaddition to the fact that the indices of refraction of these coatingsare not exactly 2 (or whatever index is needed), the chemical vapordeposition, vacuum deposition, and RF sputtering techniques for applyingthem are very expensive.

PRIOR ART

U.S. Pat. No. 2,584,905 to Moulton discloses the formation of highreflectivity coatings from alcohol solutions of TiCl₄. The index ofrefraction can be adjusted by varying the proportion of Ti to Si.Indices of refraction are obtained of only 1.45 to 1.7.

U.S. Pat. No. 2,689,858 to Boyd discloses reacting organic derivativesof orthotitanic acid in alcohol with water to prepare polymers. Theupper limit on the amount of water used is 1.6 moles per mole of Ti andthe coating is not heated at a high temperature.

U.S. Pat. No. 3,460,956 to Dahle discloses the hydrolysis of tetra-alkyltitanates in alcohol to make coatings of TiO₂. The minimum amount ofwater used is 1.5 moles per mole of Ti. An organic acid is present inthe solution.

U.S. Pat. No. 2,768,909 to Haslam and U.S. Pat. No. 2,710,267 to Boyddisclose hydrolyzing a titanium alkoxide in alcohol using atmospherichumidity.

U.S. Pat. No. 3,094,436 to Schroder discloses partially hydrolyzedesters of Ti and Si acid in alcohol. In anti-reflective coating is madeand heat is used. A silica solution is made but not a titania solution.Humidity is used to obtain titania sols.

SUMMARY OF THE INVENTION

I have discovered how to make clear solutions which contain oxideconstituents in a soluble polymerized form and from which uniform andcontinuous glass-like oxide films can be deposited on substrates atrelatively low temperatures. From these solutions quarterwaveantireflective (AR) coatings of desired thickness and refractive indexcan be deposited on photovoltaic cells and on other substrates. Thesolutions are made of a metal alkoxide, an alcohol, and water by apartial hydrolysis and polymerization process. They are applied to thesubstrate by any liquid application methods such as dipping or spraying,and the solvent and organics are evaporated. After a low temperaturebaking, the coating converts to a transparent, continuous oxide filmtenaciously bonded on the substrate. The optical quality of the filmsare equivalent or better than the presently used more expensive methodssuch as vacuum deposition.

I have also found that I can vary the index of refraction of the coatingcontinuously from 1.4 to 2.4 by using mixtures of alkoxides and bycontrolling processing variables, thus permitting fine-tuning of theindex for AR coatings on different substrates and for specificwavelength of light. Also, a coating which is anti-reflective over abroad band can be made by making the coating with two layers ofdifferent indexes or materials processed differently.

The solution and coatings of this invention are optically clear and haveabout the same physical properties as coatings deposited by priortechniques, but when optimum approach nearly the maximum reduction inreflectivity that is theoretically possible.

The coating techniques of this invention are well suited for continuousmass production of AR coating at a fraction of the cost of priortechniques. Existing process for coating solar cells cost about 20 centsa watt; this process would cost one-half to one cent per watt at presentprices.

DESCRIPTION OF THE INVENTION

FIG. 1 is an isometric view in section of a certain presently preferredembodiment of a solar cell according to the invention; and

FIGS. 2 to 9 are graphs depicting experimental results described in theExamples.

In FIG. 1, a silicon wafer 1 is composed of an N layer 2, a P layer 3,and a P⁺ layer 4 from which electricity is drawn by grids 5 and 6attached thereto respectively. On the surface of wafer 1 which isexposed to light is a coating 7 about 1/4 wavelength thick according tothis invention. The coating reduces the reflection of light back to itssource and thereby enhances the efficiency of the solar cell. Thecaptured light causes electrons to move from the N layer to the P⁺ layerthereby generating an electric current.

The coating shown in FIG. 1 is deposited from a solution derived frommetal alkoxides. Alkoxides useful in this invention have the generalformula M(OR)_(n). In the formula, "M" represents a metal ion. This ionis preferably titanium as it is commercially the most important but itcan also be tantalum. Lesser amounts of other alkoxides can be includedas dopants to alter the index of refraction of the coating. For example,up to 25% (by weight) of the M ion content can be silicon or up to 15%(by weight) of the M ion content can be another M ion that forms acompatible alkoxide, such as boron, aluminum, zirconium. The small "n"in the formula is the valence of the M ion and can vary from 3 to 5. TheR group is alkyl from C₁ to C₆. Each R group may be independentlyselected if desired though they are usually the same. The R group ispreferably ethyl, isopropyl, or butyl because these alkoxides arecommercially available and inexpensive. Also, the alcohols which areformed from these alkyl groups mix with water in the range of theinvention.

The alcohols used in preparing the coating solutions can be any liquidalcohol, although ethanol is preferred as it is inexpensive. Higheralcohols produce thicker films. The alcohol need not be the same alcoholthat is produced by the hydrolysis of the alkoxide, and in someinstances it is desirable that they be different in order to obtaindifferent viscosities. The amount of alcohol used should be sufficientto produce a solution having a solids content of about 0.1 to about 15%by weight based on equivalent TiO₂. A greater solids content for thesame degree of hydrolysis will cause the solution to gel and a lessersolids content results in undesirably thin coatings. A preferred solidscontent is about 2 to about 8% by weight based on equivalent TiO₂.Stated another way, the preferred maximum is about 1 mole alkoxide perliter of final solution.

Metal-alkoxides, in general, hydrolyze quite vigorously when broughtinto contact with water forming oxides and hydroxides as represented byreactions.

    M(OR).sub.n +nH.sub.2 O→M(OH).sub.n +nR(OH)         (1)

    M(OR).sub.n +n/2H.sub.2 O→MO.sub.n/2 +nROH          (2)

Whether the oxide or the hydroxide forms during the hydrolysis dependson the nature of the alkoxide. In either case, however, the resultantmaterial is particulate, precipitates out, and therefore is not usefulfor coating. The alkoxides of titanium and tantalum, particularly, formprecipitates even under insufficient water hydrolysis, and the formationof precipitates cannot be prevented under normal conditions. However, Ihave found a method of preparing soluble intermediate species from thesealkoxides which are capable of polymerizing into an oxide network.Precipitate formation and self-condensation which would normally occurduring the hydrolysis of these alkoxides are prevented from occurring bya careful control of molecular interaction during the hydrolysis wherecertain amounts of (OR) groups are left in the molecular structure. Thisis done by controlling first, the amount of water and dilution of thesystem, and second, by the presence of a critical amount of certainacids.

Because the water and/or alkoxide is diluted by the alcohol, theinteraction of alkoxide and water molecules are reduced to a criticalminimum where the complete hydrolysis of an alkoxide molecule cannottake place, occurrence of which would precipitate TiO₂. The firstreaction produces a partially hydrolyzed alkoxide which does notprecipitate:

    Ti(OR).sub.4 +H.sub.2 O→Ti(OR).sub.3 OH

The partially hydrolyzed alkoxide molecules include Ti(OR)₂ (OH)₂ andTi(OR)(OH)₃ and can then form a polymer with other alkoxide molecules.

    (RO).sub.3 Ti-OH+RO-Ti(OR).sub.3 →(RO).sub.3 Ti-O-Ti(OR).sub.3 +ROH

which can again react with other partially hydrolized titanium species.Because of the alkyl groups in the polymer it remains soluble in thealcohol. To keep the polymer soluble, it is necessary that not all thealkoxide bonds be hydrolyzed. Thus, the amount of water added isabsolutely critical. If the amount is less than 1.7 moles of water permole of alkoxide the coatings may be permanently cloudy and if more than4 moles of water per mole of alkoxide is used, the solution will gelfairly quickly. Preferably, the amount of water should be 1.8 to 2.2moles per mole of alkoxide.

In preparing the solution it is important to avoid contacting alkoxidewith water without diluting one or both of them with alcohol first forotherwise a precipitate will form at the point of contact. Althougheither or both the water and alkoxide can be diluted with alcohol first,it is preferable to dilute the water, then to mix the water-alcoholmixture with the alkoxide. There should be a minimum of 600 cc. ofalcohol used per mole of alkoxide; the preferable diluting range howeveris 2 to 8 liters of alcohol per mole of alkoxide.

When water and the alkoxide are brought into contact in an alcoholdiluted system, the system turns cloudy. I have found that thiscloudiness, which indicates condensation of separate particles, can betotally avoided by introducing at least 0.014 moles of either HNO₃ orHCl acids as heretofore described. If the introduction of acid is madeinto the water-alcohol solution before mixing with the alkoxide nocloudiness ever occurs, and this is therefore the preferred method ofintroducing the acid. Acid can be added anytime after the mixing occursand it will cause the cloudy slurry to turn into a clear solution.However, if more than 0.1 moles of acid are used per mole of alkoxide,the solubility of the solution is reduced and it will turn cloudy afterseveral days. So far, no organic acids have been found which work, andthe only inorganic mineral acids which have been found to work arehydrochloric acid and nitric acid, although not all acids have beentried.

The solution, once prepared, should be aged for a few hours to permitthe polymerization to proceed and stabilize. The solution should beclear at this stage. A clear solution indicates that a single continuousphase has been made rather than a particulate suspension or aprecipitate. (A gel cannot be dissolved by adding solvent.) To make acoating from the solution, it is first applied to a substrate.Application may be by spraying, painting, spinning, or other suitablemethod but dipping is the preferred method as it is most easily adaptedto mass production.

For an anti-reflective coating on a solar cell, the substrate is usuallysilicon either in the form of wafers or as a continuous strip. Sincesilicon solar cells are most sensitive at a wavelength of about 6,000 A,a one-fourth wavelength thick coating should be about 600 to about 750A. Another application of the coating is as an infrared reflectivecoating for incandescent light bulbs. By coating the inside of the bulbthe infrared light is reflected back inside the bulb which results inless electricity being used for a given light output. Other usefulsubstrates include various glasses such as gallium arsenide orcalgoginide glasses where the coating can reduce reflection.

After the substrate has been coated, it is heated to at least 300° C. todrive off solvent and organics and form an inorganic metal oxidepolymer. The film resulting after the heat treatment is continuous, hasa uniform thickness within several Angstroms, and is tenaciously bondedonto the substrate. A temperature of 350° C. is needed to drive outremaining organic groups, but it may be desirable for some applicationsto leave some organics in the coating to lower the index of refractionas is illustrated in the examples. If the material is heated above about600° C., it tends to become crystalline rather than amorphous. If theTiO₂ film thus obtained is heat treated in air it will have an index ofrefraction around 2.2. However, if the treatment is done in vacuum theindex of refraction will be around 2.4. Of course, an index ofrefraction ranging from 2.4 to all the way down to 1.4 can be obtainedby doping the solution with another suitable material. Silicon alkoxidesare suitable for this purpose. FIG. 3 gives the precise composition forany desired index between 2.4 and 1.4. The resulting clear coating hasabout the same physical properties as coatings formed by prior vacuumtechniques, but its optical properties are superior because the bestcoatings reduce reflectivity to very nearly the maximum amount which istheoretically possible.

When a single coating is applied to a substrate, the reflectivityreduction occurs at a minimum wavelength. The minimum wavelength can bebroadened to cover a wide range of wavelengths by coating the substratewith two layers of film having different indices. For silicon, the lowerlayer should have a index of about 2.4 and the top layer a index ofabout 1.4. The lower layer index of 2.4 can be obtained by coating with100% TiO₂ in vacuum. The top layer index of 1.4 can be obtained bycoating with a 90%-10% mixture of SiO₂ and TiO₂ in air. Both coatingsshould each be a quarter wavelength thick. Each layer is firedseparately.

The following examples further illustrate this invention.

EXAMPLE 1

18 g of H₂ O and 0.7 g HNO₃ were mixed well into 1000 g of ethylalcohol. To this 114 g (1/2 mole) titanium tetra-ethoxide Ti(OC₂ H₅)₄was gradually added. A clear solution resulted. This solution wasallowed to stand several hours. A dendritic-web silicon ribbon having1/2 inch width was immersed in it. This sample was then withdrawn fromthe solution at a rate of 40 cm/min and baked in air at 400° C. for afew minutes. A uniform antireflective film of titanium oxide having athickness of 680 A and refractive index of 2.12 was formed on thesilicon ribbon.

EXAMPLE 2

18 g of H₂ O was mixed well into 1000 g of ethyl alcohol. To this 170 g(1/2 mole) titanium butoxide, Ti(OC₄ H_(g))₄ was added under stirring.Within a few minutes the entire solution turned cloudy. To this 0.9 g of70% concentrated nitric acid was added (0.02 mole HNO₃ per mole ofalkoxide) with stirring. The milky slurry became clear within minutes.This solution was then spin applied on Czochalski-type round siliconwafers 11/4" in diameter at 900 rpm. When these wafers were fired in airthey had a coating with a refractive index slightly over 2.1 and athickness around 640 A. Similarly coated wafers heat treated in vacuumhad a refractive index around 2.4.

EXAMPLE 3

0.36 g (0.02 moles) H₂ O was mixed into 27 g ethyl alcohol. Into this,4.06 g tantalum alkoxide, Ta(OC₂ H₅)₅, was introduced under mixing. Aclear liquid resulted. After 10 minutes of standing at room temperature,this solution acquired a milky appearance. 1 drop of HNO₃ acid wasintroduced into the solution and mixed. The cloudy appearancedisappeared. It contained 7% equivalent Ta₂ O₅ by weight.

After 3 days of aging, the solution described above was applied on asilicon solar cell with a spin application at 2500 rpm. The coated cellwas then baked at 400° C. for 5 minutes. The following test results showthe performance of the cell before and after the coating.

    ______________________________________                                                  I.sub.sc (ma)                                                                          V.sub.oc (V)                                                                          Efficiency (%)                                     ______________________________________                                        Before Coating:                                                                           21.20      0.539   8.95                                           After Coating:                                                                            28.6       0.552   12.7                                           ______________________________________                                         I.sub.sc = short circuit current                                              V.sub.oc = voltage (open circuit)                                             ##STR1##                                                                 

EXAMPLE 4

15 g H₂ O, 15 g silicon tetraethoxide (Si(OC₂ H₅)₄), and 0.6 g nitricacid were mixed into 853 g ethyl alcohol. Into this 116 g of titaniumisopropoxide (Ti(OC₃ H₇)₄) was added gradually. A clear solution having3.7 weight percent equivalent titania concentration resulted. Similarly,by using larger quantities of ethyl alcohol, solution having 0.5, 1.0,and 2.0 weight percent equivalent titania concentrations were alsoprepared. One-half inch wide 3 ft. long stainless steel ribbons wereimmersed in these solutions after 1 day aging. These samples were thenwithdrawn from the solutions at various speeds up to 64 ft/min. Theywere baked in air at 400° C. for 5 minutes. The coating thicknesses werethen measured. FIG. 2 shows the effect of solution concentration andwithdrawal rates on the thickness of films.

EXAMPLE 5

The following two solutions were prepared:

Solution A: 20.8 g silicon tetraethoxide, Si(OC₂ H₅)₄ was mixed with 148g ethyl alcohol. Precisely 1.8 g H₂ O was also added along with 1 dropof HNO₃ and the mixture was refluxed at 60° C. for several hours thenset aside to cool. This gave a partially hydrolyzed silanol solutioncontaining 3.5 wt.% equivalent SiO₂.

Solution B: 22.8 g Ti(OC₂ H₅)₄ was added to 102 g ethyl alcohol. 3.5 gH₂ O and 4 drop of HCl acid were added to 100 g ethyl alcohol. These twopreparations were mixed and aged several hours. The resultant liquid hada 3.5 wt.% equivalent TiO₂.

The two solutions were then mixed in various proportions and werepermitted to age for several hours. The mixtures were deposited onsilicon wafers by spinning at 1000 rpm and were heated in air and invacuum at 500° C. to form coatings about 600 A thick. FIG. 3 shows thatthe index of refraction depends on the proportion of TiO₂ to SiO₂ and onwhether the coating was fired in air or in vacuum. A vacuum produces adenser coating having a higher index of refraction.

EXAMPLE 6

Example 5 was repeated except that Solution B contained 3.5 equivalentof TiO₂. The solution was placed on a silicon wafer and centrifuged atspeeds ranging from 500 to 5,000 rpm before being heated at 500° C. FIG.4 shows the effect of centrifuge speed on film thickness.

EXAMPLE 7

Example 6 was repeated at a centrifuge speed of 2000 rpm, except thatthe solutions were then baked at 250° C. to 500° C. FIG. 5 shows theeffect that bake temperature has on film thickness and index ofrefraction. FIG. 6 shows the reflectivity of the surface films baked atvarious temperatures. It also shows how the minimum reflectivity can beshifted at various wavelengths with bake temperatures.

EXAMPLE 8

Example 6 was repeated except that the solution was 3.0% equivalent TiO₂(2 moles H₂ O), the alcohol was a mixture of ethanol and tertiarybutanol, and the wafer was spun at 2,000 rpm. FIG. 7 shows the effect onfilm thickness of various proportions of ethanol and t-butanol. FIG. 7shows that the thickness of the film can be controlled by the type ofalcohol used.

EXAMPLE 9

Example 6 was repeated using 2.5 moles of water per mole of Ti(OC₂ H₅)and controlling equivalent TiO₂ concentration by dilution with ethanol.Solutions were then coated on silicon wafers at 2000 rpm and baked at500° C. FIG. 8 shows the relationship between solution concentration andfilm thickness. This gives another method of controlling film thickness.

EXAMPLE 10

P+ back surface field silicon solar cells having active areas slightlyover 1 cm² were employed for anti-reflective testing. The silica andtitania solutions were prepared as described in Example 5. Thesesolutions were mixed to prepare a third solution to coat an oxide filmwith an index of 2 as indicated by FIG. 3 which requires 88% of TiO₂ and12% of SiO₂. The fine tuning of the coating thickness is done by varyingthe rpm in spin application and the drawing speed in the dippingapplication. The solutions used required a spin speed of 800-1,000 rpm.The dipping was done by immersing the entire sample into the coatingliquid and pulling it out at a continuous speed. It was found thatextremely uniform anti-reflective coatings are deposited at a pullingrate 1-2 ft/min. It also appears to be feasible to increase this speedan order of magnitude or more by using diluted solutions. The followingtable gives the results.

    ______________________________________                                        Solar  I.sub.sc *                                                                             (mA)    Efficiency                                                                            (%)***                                        Cell   Uncoated Coated  Uncoated                                                                              Coated                                                                              Improvement                             ______________________________________                                        A      21.10    29.40   8.56    12.24 43%                                     B      21.60    31.00   9.37    13.34 42%                                     C      21.40    30.50   9.03    12.91 43%                                     D      20.80    28.90   8.65    12.82 48%                                     E      22.00    30.50   9.43    14.04 49%                                     ______________________________________                                         * = Short circuit current                                                     *** = Calculation of efficiency takes into account changes in open circui     voltage and fill factor which are relatively insensitive to AR coatings       and therefore not given here separately.                                 

The E cell was a doubly-coated cell with films of two different indices.The rest of the cells are coated with single anti-reflective film withan index of 2. As shown, these cells showed as much as 48% increase inthe efficiency.

From a purely theoretical point of view, if the reflection were to beeliminated entirely, there would be about 60% more energy available tothe device over the uncoated state (38/62=60%). However, this sixtypercent is unattainable as an increase in device electrical output forseveral reasons. First, the zero reflectivity occurs only at onewavelength, not throughout the entire spectrum. Secondly, the responseof the silicon device varies with the wavelength; therefore, it will notrespond to all of the energy gain. For these reasons, the actual limiton potential efficiency increase with an anti-reflective coating isestimated to be about 50%. The 48 to 49% increase in efficienciesrecently attained in this work appears to approach this limit.

EXAMPLE 11

Single coatings produce minimum reflectivity at a wavelength given byλ=4n_(c) t_(c) where n_(c) and t_(c) are the index and thickness of thecoating respectively. All other wavelengths, however, are reflected withincreasing intensity as they deviate from this value. Since inphotovoltaic silicon cells the energy conversion takes place in therange of wavelengths covering approximately the solar spectrum, it ishighly desirable to produce coatings whose antireflectivecharacteristics also cover the entire range of solar spectrum.

It has been demonstrated that quarterwave double AR coatings can also bedeposited by the technique of this invention.

A silicon wafer was initially coated with a quarter wave thick TiO₂ filmfired in vacuum and having an index of 2.4. The solution used wassimilar to that of Example 1. Then a second quarter wave film having acomposition of 90% SiO₂ -10% TiO₂ was deposited from a solution preparedsimilarly to Example 5. As predicted by FIG. 3 this second coating had arefractive index of 1.4.

The combined reflectivity of this double film is shown in FIG. 9. As waspredicted, a wide spectrum of antireflectivity was obtained. The totalreflectivity as integrated over the 4500 A to 900 A range was less than3% (uncoated silicon surfaces reflect about 40% in this range). A solarcell coated with double film showed 49% improvement in efficiency (see"solar cell E" in Example 10).

What is claimed is:
 1. A substrate having an oxide coating thereon whichhas a predetermined index of refraction between about 1.4 and about 2.4made by(A) preparing a clear solution which comprises:(1) alkoxidehaving the general formula M(OR)_(n) where M is selected from the groupconsisting of 0 to 100% Ti, 0 to 25% Si, 0 to 100% Ta, and up to 15% ofa metal ion which forms an alkoxide, R is alkyl from C₁ to C₆, and n isthe valence of M; (2) about 1.7 to about 4 moles of water per mole ofalkoxide; (3) sufficient alcohol to give a solids content of about 0.1to about 15% and (4) a sufficient amount of a suitable acid to preventcloudiness; (B) applying said clear solution to said substrate; and (C)heating said substrate to about 300° to about 600° C.
 2. A coatedsubstrate according to claim 1 wherein said substrate is silicon.
 3. Acoated substrate according to claim 2 wherein said silicon is part of asolar cell.
 4. A coated substrate according to claim 1 wherein saidcoating is about 600 to about 750 A thick.
 5. A coated substrateaccording to claim 1 wherein said substrate is the inside of anincandescent light bulb.
 6. A coated substrate according to claim 1wherein M is a mixture of Ti and Si.
 7. A coated substrate according toclaim 1 wherein M is all Ti.
 8. A coated substrate according to claim 1wherein said solution is aged then applied to said substrate by dipping.9. A coated substrate according to claim 1 wherein R is selected fromthe group consisting of ethyl, isopropyl, butyl, and mixtures thereof.10. A coated substrate according to claim 1 wherein steps (B) and (C)are repeated to produce a two-layered coating.
 11. A coated substrateaccording to claim 1 wherein the amount of said acid is less than 0.1moles per mole of said alkoxide.
 12. A coated substrate according toclaim 11 wherein said acid is nitric or hydrochloric.